The invention relates generally to photoconductive image forming, and more particularly to photoconductive image forming utilizing photoconductive toner deposited on an image forming substrate.
There are several conventional photoconductive image forming methods, such as, Sugarman's method in U.S. Pat. No. 2,758,939 which is conventional, and does not use photoreceptors. Accordingly, it is easier to form color images with Sugarman's method than with electrophotographic methods of electrostatically forming images by use of photoreceptors and either insulating or electroconductive dry toners.
In Japanese unexamined application No. 60-138566 by Toshiba Electric Co. image formation is by forming a thin layer of photoconductive toner which is negatively charged with carriers on the entire surface of a transparent and electroconductive rotating hollow substrate by a magnetic brush. The toner layer is exposed to an image which is projected from the inside of the hollow substrate. The exposure reduces the resistance of the exposed toner so that static positive charges are applied to the exposed toner while a bias voltage is applied. The positively charged toner particles are transferred to a recording paper by electric field inducement.
This method has drawbacks since it is difficult to form a thin, controlled layer of photoconductive toner over the entire surface of the electroconductive substrate. Further, since the exposed toner is transferred to the transfer paper at the same time as the exposure, unexposed toner also contacts the transfer paper. This results in this unexposed toner being transferred to the transfer paper, resulting in images with undesirable background fog.
Additional transfer methods have been proposed in Japanese unexamined application Nos.: 60-205469, 60-205471, 61-17155, 61-17156 and 61-18970-18974, proposed by Konishiroku Co. According to this method, photoconductive toners and carriers are formed into a "magnetic brush". A direct image exposure is applied from above the magnetic brush and unexposed photoconductive toner is caused to fly to a counter electrode substrate and then transferred onto transfer paper.
This method also has shortcomings. The unexposed toner which flies to the conducting substrate causes unavoidable scattering. Therefore, it is difficult to obtain suitably clear images. Further, the method described in the aforementioned patents involve an excessive number of image forming steps. This increases the size, complexity and cost of an apparatus for practicing this method.
An image forming method utilizing simultaneous exposure and toner development which does not utilize photoconductive toners was proposed in Japanese unexamined application No. 58-153957. During exposure the surface of a photoreceptor is rubbed with a brush of electroconductive magnetic toner to which a bias voltage is applied. The amount of electrostatic charge applied to the electroconductive magnetic toner in contact with the surface of the photoreceptor varies greatly between the unexposed area of the photoreceptor which functions as an insulator and the exposed area which acts as a conductor. The toner image is formed by utilizing the differences in the charge between toner corresponding to exposed portions and nonexposed portions to transfer the image to a transfer medium.
This image forming method also has drawbacks. It is undesirable to incorporate photoreceptors into an image forming apparatus. Secondly, transferring toner to recording paper by this method does not transfer toner to the paper properly. During corona transfer, the toner charges are neutralized during the short relaxation time due to their electroconductive properties. This decreases their residual charge and thereby decreases their electrostatic attraction to the recording paper.
Conventional toners proposed for use in photoconductive image forming methods are not fully satisfactory. These toners generally have a basic composition and include inorganic material such as dye-sensitized ZnO, dye-sensitized TiO.sub.2 or organic photoconductive agents, such as phthalocyanine, quinacridone and benzidine as well as binders and colorants. Examples of conventional dye-form photoconductive agents are described in Japanese unexamined application No. 61-230154-230157 by Ricoh Co. Toners in which the photosensitive wave length has been extended from the visible region to near infrared wave lengths (400 nm-750 nm) have been described in Japanese unexamined application Nos. 61-9657 and 61-34554 by Toshiba Electric Co. Further, Japanese unexamined application No. 59-78358 describes the photoreceptor sensitization of ZnO the typically utilized photoconductive agent, to the near infrared wave length region.
These conventional toners are not completely acceptable. The choice of photoconductive agent, colorant and sensitizer is dependent on the selected light source which complicates the production process and increases toner costs. The photosensitivity and electrical properties of the toners is reduced when they are blended. This is especially unsuitable when mixed photoconductive agent and black colorant is used. Furthermore, when carbon black is used as the black colorant, because the absorption region is extended from the visible region to the infrared region, the photosensitivity of photoconductive toners is significantly reduced.
Inexpensive semiconductor lasers expose in the near infrared region Because conventional photoconductive toners cannot be effectively sensitized to the near infrared wave length region, it is difficult to use inexpensive semiconductor lasers for the writing light source. This increases the cost of the apparatus.
Transfer of color images with a photoconductive toner method is described in Japanese unexamined application No. 58-114043. Three colored photoconductive toners are mixed. A layer of toners is formed on a roller and exposed and charged simultaneously through a transparent electrode and transferred to a recording sheet by a transfer roller. Additionally, Japanese unexamined application No. 60-31150 (Sony Corporation) proposes that three colored photoconductive toners are mixed and a layer of the mixed toners is formed on a conductive substrate. The substrate is exposed three separate times from above the photoconductive toner layer. Exposure creates differences in charge between exposed and nonexposed toners are separated to form color images. Because it is difficult to form a single layer of photoconductive toners with conventional image forming methods and colored toners, the conventional color methods are also not fully acceptable. Problems include poor color reproduction and poor image quality.
Accordingly, it is desirable to provide for photoconductive imaging forming which does not suffer from these shortcomings of the prior art.